Find the sum : (-5) + (-8) + (-11) +....... + (-62).

Question

KnowledgeBoat · Accepted Answer

Let total number of terms in an A.P. be n. We know that, ∴ an = a + (n - 1)d Given, a = -5 d = -8 - (-5) = -3 ⇒ an = -5 + (n - 1)-3 ⇒ -62 = -5 + (n - 1)-3 ⇒ -62 - (-5) = (n - 1)-3 ⇒ -62 - (-5) = (n - 1)-3 ⇒ -57 = (n - 1)-3 ⇒ = (n - 1) ⇒ 19 = (n - 1) ⇒ n = 19 + 1 ⇒ n = 20 We know that, Sum of n terms of an A.P. is given by, ∴ Sn = (a + l) ⇒ S20 = [-5 + (-62)] = 10 [-5 + (-62)] = 10(-67) = -670. Hence, S20 = -670.

Mathematics

Find the sum :
(-5) + (-8) + (-11) +……. + (-62).

AP

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